For optical disks, various material and systems have been conventionally developed, and further development of new materials and systems is expected. Accordingly, it is required that optical disks to be developed can be used compatibly with various optical disks including ones may be developed in future. For this reason, it is recommended to record the informations required for the reproducing apparatus onto respective disks. Such a pre-recorded region is a PEP (Phase Encoded Part) as described later in detail and is provided on the innermost circumferential portion of the disk.
The recording content of the PEP is, e.g., data coding format, tracking servo format, error correction code, light reflection coefficient on the reference plane, kind of medium, information for initially setting the reproducing apparatus such as the maximum value of the light beam at the time of reproduction, and CRC (Cyclic Redundancy Check) data for checking whether or not data obtained by reading such information is correct. They are recorded in accordance with a predetermined format.
In using disks, it is required to first read this portion to initially set the reproducing apparatus so that it is in an optimum state in conformity with the disk specification thereof. However, it is impossible to know the recording system until such a portion is read out. For this reason, there is employed a recording structure such that readout operation is possible even in the state where no tracking servo is applied to the reproducing apparatus (as a matter of course, there is some eccentricity). However, under the circumstances of actual use, the influence of dust or scratches on the disk surface would be anticipated (see "Collected Materials at Optical Disk Society Meeting in 1987" March, 1987, Society of the promotion for optical industrial technology).
FIG. 6 is an explanatory view of the format of a direct read after write (DRAW) optical disk. An example in conformity with the ISO standard is illustrated. As shown in this figure, each area in a radial direction of the optical disk 2 is divided according to use. Roughly, this area is divided into the control track area and the user area 12. A range of a radius of 30 to 60 mm is allotted to the user area 12, and various control tracks are arranged in other areas of a radius 29 to 30 mm and a radius of 60 to 60.5 mm. Namely, the PEP area 4 is arranged in the range of 29 to 29.5 mm, a SFP transition area 6 is arranged in the range of 29.5 to 29.52 mm, a SFP area 8 is arranged in the range of 29.52 to 29.7 mm, a medium manufacturer's area 10 is arranged in the range of 29.7 to 30 mm, and an external circumferential SFP 14 is arranged in the range of 60.15 to 60.5 mm. In this example, the control track area is composed of the PEP area 4 recorded in accordance with the format capable of reading by all drivers irrespective of the servo format of the optical disk, and a SFP area 8 recorded in accordance with the same format as that of the user area 12.
The information written into the PEP is information for setting the drive condition of the optical disk so that the SFP can be read.
A disk player serving to record information onto such an optical disk or reproduce them therefrom, first obtains control information such as the effective surface, or the range of reflection coefficient, etc. by the discriminator of the cartridge for accommodating the optical disk to read the PEP area 4 after setting of the control. In the PEP area 4, information such as the servo system, rotation mode, modulation system, ECC (error correction code), sector size, coefficient of reflection, signal polarity level, land/groove, reading power, media type, etc. are described. The disk player judges, on the basis of the above information, whether or not an optical disk can be use d f or recording/reproduction, and when it is judged that the optical disk concerned can be used, the disk player carries out setting of the operation by this control information. In order that these information can be read by players of any type, data are recorded, in accordance with the phase modulated format, by pits called ISO pits having a narrow pitch of about 1.mu. independent of the servo system and the land/groove system.
FIG. 7 is a block diagram showing an outline of a conventional apparatus for reading the optical disk of the format shown in FIG. 6. As shown in this figure, an optical disk 2 is rotationally driven by a spindle 16. At the same time, read operation of information on the optical disk 2 is conducted by an optical pick-up 18. The positional control in a radius direction of the optical disk 2 is carried out by moving the optical pick-up 18 at a high speed by means of a linear motor 20. In addition, a fine positional control of the optical pick-up 18 is carried out by delivering, to the linear motor 20, a linear motor control signal (pulsated drive signal) output from a control unit 24.
The linear motor 20 is identically considered as a d.c. motor. When the linear motor 20 is driven a double or bipolar pulse waveform as shown in FIG. 8(a), a velocity v of the optical pick-up 18 such that acceleration/deceleration can be smoothly conducted is provided as shown in FIG. 8(b). As a result, the optical pick-up 18 can be stopped under the state of displacement x as shown in FIG. 8(c). Accordingly, minute or fine positional alteration of the optical pick-up 18 can be carried out by applying a pulse to the linear motor as a linear motor control signal 36.
The alteration sequence of the read area briefly referred to the above will be now described in detail with reference to FIG. 9.
Initially, positioning of the optical pick-up is mechanically completed by the inner circumferential position sense switch 32 of FIG. 7 (S11). Then, the PEP signal is read (S12) and the CRC error is checked (S13). When the error is detected, there will be two cases. First case is that the read position is in correct, and is thus positioned at the boundary of the PEP area. Second case is that the read position is correct. In the first case, since the level signal is detected (S14), the process shift to a limit control operation for returning from the boundary area (S15). In the second case, since it is considered that the read position is not beyond the PEP area, but error occurs by small defect, etc., it is sufficient to slightly alter or change the read position. The alteration method is carried out by applying a pulse to the linear motor as shown in FIG. 8. As an example, such a method to move the read position in an outer circumferential direction step by step is used. This method corresponds to the fine adjustment of position of the flowchart in FIG. 9 (S16). After that operation, an operation sequence follows to carry out read operation of the PEP for a second time (S12) to carry out checking of the CRC error.
Furthermore, in order to allow the light beam from the optical pick-up 18 to be positioned in the control track PEP area 4 on the inner peripheral side of the optical disk 2, inner circumferential position sense switch 32 is provided. By inputting, to the control unit 4, an inner peripheral position sense signal 34 that is output from the inner circumferential position sense switch 32, a linear motor control signal 36 is output to move the linear motor 20. Thus, positioning of the optical pick-up 18 is carried out.
A read signal 18A from the optical pick-up 18 is input to the signal sense unit 22. A signal in the control track PEP area 4 is written three times with one round being divided into three sectors. For a demodulated signal thereof, a direction signal 28 and a level sense signal 30 are included in addition to a data signal 26. On the basis of these signals, the control unit 24 conducts a tracking control in reading data in the PEP area 4 and in reading data in the user area 12 through the linear motor control signal 36.
The PEP area 4 has a width of 500.mu. in a radial direction. Since the specification of eccentricity of the optical disk 2 is 50.mu. the signal can be satisfactorily read as long as all standards are completely satisfied.
As stated above, the PEP area 4 of the optical disk 2 is recorded onto the portion of 29.0 to 29.5 mm in the vicinity of the inner most circumference of the optical disk 2. This area 4 is formed so that off track reading, i.e., reading system capable of effecting read operation can be implemented with a pitch narrower than an ordinary track pitch called an ISO pit within the width of 500.mu.. Since an allowed quantity of eccentricity of the optical disk 2 is determined to fall within 50.mu. as required by the standard, if a control can be conducted such that the innermost circumferential position of the optical pick-up 18 is correctly equal to 29.25 mm, a track of 500.mu. in width can be precisely read without any problems. This innermost circumferential position is sensed by the inner peripheral position sensor switch 32.
Since the conceivable disk read position control is conducted as above, there are problems as described below.
In order to precisely read the PEP area 4 on the optical disk 2, it is required to effect control such that the innermost circumferential position of the optical pick-up 18 is precisely equal to 29.25 mm. The positioning accuracy at this time all depends upon the sense accuracy of the inner circumferential position sense switch 32. In addition, it is required that the accuracy in the positional control of the optical pick-up 18 is maintained at all times. For this reason, as the inner circumferential position sense switch 32, it is necessary to use a switch having a high sense resolution and a less change with the lapse of time. Further, in the case of using a switch which is simply turned on or off at the sense position, such an adjustment is very troublesome. Namely, the problem that cost is extremely increased results.
In reading the PEP area 4 of the optical disk 2, it is inevitable that inconvenience in read operation occurs in practice by the reproducing condition and the optical disk 2. As a result, error in reading data may occur by the failure of the signal read operation, etc. In this case, it is required to conduct control to alter the read position on the optical disk 2 to retry another read operation. In such a case, since the PEP area 4 has a broad width and the read operation is conducted on a narrow track, there is the problem that tracking control by the track cross signal cannot be carried out.
On the other hand, it is proposed to use a switch capable of sensing position with a high resolution as the position sense switch 32. However, when such a switch is employed, there is the problem that not only the merit obtained with the direct access for conducting a tracking control only by the track cross signal in the case of providing an access to the user area 12 is reduced, but also the cost is extremely increased by the necessity of using an external scale.